In a Caribbean mangrove forest, scientists discovered a species of bacteria that grows to the size and shape of a human eyelash.
These cells are the largest bacteria ever observed, thousands of times larger than the best-known bacteria such as Escherichia coli. “It would be like meeting another human the size of Mount Everest,” said Jean-Marie Volland, a microbiologist at the Joint Genome Institute in Berkeley, California.
Dr. Volland and his colleagues published their study of the bacterium, called Thiomargarita magnifica, on Thursday in the journal Science.
Scientists once thought that bacteria were too simple to produce large cells. But Thiomargarita magnifica turns out to be remarkably complex. With most of the bacterial world still to be explored, it is entirely possible that even larger and even more complex bacteria are waiting to be discovered.
It’s been about 350 years since Dutch lens grinder Antonie van Leeuwenhoek discovered the bacteria by scraping his teeth. When he placed dental plaque under a primitive microscope, he was surprised to see single-celled organisms swimming around. Over the next three centuries, scientists found many other types of bacteria, all invisible to the naked eye. An E. coli cell, for example, measures about two microns, or less than ten thousandths of an inch.
Each bacterial cell is its own organism, which means it can grow and divide into a pair of new bacteria. But bacterial cells usually live together. Van Leeuwenhoek’s teeth were coated with a gelatinous film containing billions of bacteria. In lakes and rivers, some bacterial cells join together to form tiny filaments.
We humans are multicellular organisms, our bodies are made up of about 30 trillion cells. Although our cells are also invisible to the naked eye, they are typically much larger than those of bacteria. A human egg can reach about 120 microns in diameter, or five thousandths of an inch.
Cells from other species can grow even larger: the green alga Caulerpa taxifolia produces blade-shaped cells that can grow up to a foot.
As the chasm between small and large cells emerged, scientists looked to evolution to make sense of it. Animals, plants and fungi belong to the same evolutionary lineage, called eukaryotes. Eukaryotes share many adaptations that help them build large cells. The scientists reasoned that without these adaptations, bacterial cells would have to remain small.
For starters, a large cell needs physical support so it doesn’t fall apart or fall apart. Eukaryotic cells contain rigid molecular threads that function like poles in a tent. Bacteria, however, do not have this cellular skeleton.
A large cell also faces a chemical challenge: As its volume increases, it takes longer for molecules to move around and find the right partners to carry out precise chemical reactions.
Eukaryotes have developed a solution to this problem by filling cells with tiny compartments where distinct forms of biochemistry can take place. They keep their DNA coiled up in a bag called the nucleus, along with molecules that can read genes to make proteins, or the proteins make new copies of DNA when a cell reproduces. Each cell generates fuel inside pockets called mitochondria.
Bacteria lack the compartments found in eukaryotic cells. Without a nucleus, each bacterium normally carries a free-floating loop of DNA within it. They also lack mitochondria. Instead, they normally generate fuel with molecules embedded in their membranes. This arrangement works well for tiny cells. But as the volume of a cell increases, there is not enough space on the cell’s surface for enough fuel-generating molecules.
The simplicity of bacteria seemed to explain why they were so small: they simply lacked the essential complexity to grow.
However, that conclusion was made hastily, according to Shailesh Date, founder of the Complex Systems Research Laboratory in Menlo Park, Calif., and co-author with Dr. Volland. Scientists have made sweeping generalizations about bacteria after studying only a small portion of the bacterial world.
“We’ve just scratched the surface, but we’ve been very dogmatic,” he said.
That dogma began to crack in the 1990s. Microbiologists discovered that some bacteria evolved independently in their own compartments. They also discovered species that were visible to the naked eye. Epulopiscium fishelsoni, for example, came to light in 1993. Living inside the surgeonfish, the bacteria grow up to 600 microns in length – larger than a grain of salt.
Olivier Gros, a biologist at the University of the Antilles, discovered Thiomargarita magnifica in 2009 while researching the mangrove forests of Guadeloupe, a cluster of Caribbean islands that are part of France. The microbe looked like miniature pieces of white spaghetti, layering on dead tree leaves floating in the water.
At first, Dr. Gros didn’t know what he had found. He thought the spaghetti might be fungi, tiny sponges, or some other eukaryote. But when he and his colleagues extracted DNA from samples in the lab, it turned out to be bacteria.
Dr. Gros joined forces with Dr. Volland and other scientists to take a closer look at the strange organisms. They wondered if bacteria were microscopic cells joined in chains.
That turned out not to be the case. When the researchers peered into the bacterial noodles with electron microscopes, they realized that each was its own giant cell. The average cell measured about 9,000 microns in length, and the largest was 20,000 microns – enough to span the diameter of a penny.
Studies on Thiomargarita magnifica advanced slowly because Dr. Vallant and his colleagues have yet to figure out how to grow the bacteria in their lab. For now, Dr. Gros needs to collect a fresh supply of the bacteria every time the team wants to run a new experiment. He can find them not just in leaves, but in oyster shells and plastic bottles on the sulfur-rich sediments in the mangrove forest. But bacteria seem to follow an unpredictable life cycle.
“For the last two months, I can’t find them,” said Dr. gross “I don’t know where they are.”
Inside the cells of Thiomargarita magnifica, the researchers discovered a bizarre and complicated structure. Their membranes have many different types of compartments built into them. These compartments are different from those in our own cells, but they can allow Thiomargarita magnifica to grow to enormous sizes.
Some of the compartments appear to be fuel-generating factories, where the microbe can harness energy in nitrates and other chemicals it consumes in the mangroves.
Thiomargarita magnifica also has other compartments that look remarkably like human cores. Each of the compartments, which scientists call cucumbers for the tiny seeds of fruits like kiwis, contains a DNA loop. While a typical bacterial cell has only one DNA loop, Thiomargarita magnifica has hundreds of thousands of them, each inside its own cucumber.
Even more remarkable, each cucumber contains factories for building proteins from its DNA. “They essentially have small cells within cells,” said Petra Levin, a microbiologist at Washington University in St. Louis, who was not involved in the study.
Thiomargarita magnifica’s huge supply of DNA can allow it to create the extra proteins it needs to grow. Each cucumber can make a special set of proteins needed in its own region of the bacterium.
Dr. Volland and his colleagues hope that once they start culturing the bacteria, they will be able to confirm these hypotheses. They will also tackle other mysteries, such as how the bacterium can be so resilient without a molecular skeleton.
“You can take a single strand out of the water with tweezers and put it in another container,” Volland said. “How does it maintain itself and how does it acquire its shape – these are questions we don’t answer.”
The Doctor. Date said there could be more giant bacteria waiting to be found, perhaps even larger than Thiomargarita magnifica.
“How big they can get, we don’t really know,” he said. “But now, this bacterium has shown us the way.”